1. Field of the Invention
The present invention relates to a silyl-substituted cyclometalated transition metal complex and an organic electroluminescence device using the same, and more particularly, to a silyl-substituted cyclometalated transition metal complex capable of emitting light over a wide range from a blue region to a red region from the triplet metal-to-ligand charge transfer (MLCT) state and having good thermal stability and an organic electroluminescence device using the same as an organic layer forming material.
2. Description of the Related Art
Generally, an organic electroluminescent (hereinafter referred to as EL) device is a spontaneous light-emitting display device which emits light by the energy generated through the recombination of electrons and holes when an electric field is applied to thin films made of fluorescent or phosphorescent organic compounds (hereinafter referred to as organic layers), and provides various advantages suitable to be used for portable electronic devices, including lightness, constructional simplicity, high quality, wide viewing angle, high color purity, perfect implementation of motion pictures, low power consumption, a low driving voltage, and so on.
A general organic EL device includes an anode, a hole transport layer, an emission layer, an electron transport layer, and a cathode, sequentially formed on a substrate. The hole transport layer, the emission layer, and the electron transport layer are organic layers made of organic compounds. The organic EL device having the above-described configuration is driven as follows. When a voltage is applied between the anode and the cathode, holes injected from the anode migrate to the emission layer via the hole transport layer. Electrons emitted from the cathode are injected into the emission layer via the electron transport layer. The electrons and the holes recombine in the emission layer to generate excitons. While the excitons radioactively decay, light corresponding to a band gap of the molecules is emitted.
Materials forming the emission layer of the organic EL device are classified into a fluorescent material that uses singlet excitons and a phosphorescent material that uses triplet excitons, according to a light-emitting mechanism. The fluorescent material or the phosphorescent material forms an emission layer by itself or by being doped into an appropriate host material. As a result of the electron excitation, singlet excitons and triplet excitons are produced in the host. Statistically, the singlet excitons and the triplet excitons in an organic EL device are created in a ratio of about 1:3.
Organic EL devices using a fluorescent material as a material for forming an emission layer are disadvantageous in that triplets are consumed from the host. However, organic EL devices using a phosphorescent material as a material for forming an emission layer are advantageous in that singlet excitons and triplet excitons are both utilized to achieve the internal quantum efficiency of 100%. Thus, an organic EL device using a phosphorescent material as a material for forming an emission layer has a high luminescence efficiency compared with an organic EL device using a fluorescent material.
Introduction of a heavy metal such as Ir, Pt, Rh, or Pd to organic molecules has led to spin-orbital coupling due to a heavy atom effect so that a triplet state and a singlet state coexist, enabling a forbidden transition, thereby allowing phospholuminescence to occur even at room temperature.
More recently, developments have led to the discovery of highly efficient green and red luminescent materials using photoelectroluminescence of up to 100%.
As highly efficient luminescent materials using phospholuminescence, various materials employing various transition metal compounds containing a transition metal such as iridium or platinum, have been being reported. However, materials satisfying requirements for realizing a full-color display of high efficiency or white electroluminescence at low power consumption are only restricted to ones emitting in the green and red ranges, and enough blue phosphorescent materials have not been reported, which is becoming a barrier to the development of phospholuminescent full-color display devices.
To address the above-described problems, intensive development of blue emission materials is under way (International Patent Publication No. WO 02/15645 A1, U.S. Patent Publication No. 2002/0064681 A1). Also, there have been proposed organometallic complexes having a bulky functional group or a functional group having a high intensity ligand field, e.g., a cyano group, introduced thereto to increase a difference between HOMO-LUMO energy levels by transforming the molecular geometry (Mat. Res. Soc. Symp. Proc. 708, 119, 2002; 3rd Chitose International Forum on Photonics Science and Technology, Chitose, Japan, 6-8 Oct., 2002).
In addition, a cyclometalated transition metal complex composed of nitrogen atoms and carbon atoms and an organic EL device including the same are disclosed in U.S. Patent Publication No. 2002/0134984 A1. However, all of these materials do not show satisfactory physical properties in color purity, luminescence efficiency, lifespan, thermal stability, etc.